1. Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to, an image forming apparatus including a power source that outputs a superimposed bias in which a direct current (DC) voltage is superimposed on an alternating current (AC) voltage.
2. Description of the Related Art
Image forming apparatuses equipped with a transfer bias output device that outputs a superimposed bias as a transfer bias in which an alternating current bias and a direct current bias are superimposed are known. In the image forming apparatuses of this kind, toner images formed on photoconductors through known electrophotographic process are primarily transferred onto a belt-type intermediate transfer member (hereinafter, intermediate transfer belt) and then secondarily onto a recording medium in a secondary transfer nip at which a contact roller contacts a front surface of the intermediate transfer belt. A back surface roller contacts a back surface of the intermediate transfer belt so as to interpose the intermediate transfer belt between the contact roller and the back surface roller.
In order to secondarily transfer the toner image through known electrostatic transfer process, a secondary transfer bias is applied to the back surface roller while the back surface roller contacts the back surface of the intermediate transfer belt. In order to enhance secondary transfer ability, a superimposed bias, in which an AC voltage and a DC voltage are superimposed, is output as the secondary transfer bias. In other words, the secondary transfer bias is a superimposed bias. The intermediate transfer belt is formed of multiple layers including a base formed into an endless loop on which a top layer having greater elasticity than the base is laminated.
In this configuration, while the durability of the intermediate transfer belt is maintained depending on the durability of the base, the elastic top layer of the intermediate transfer belt can tightly contact recessed portions of an uneven surface of paper such as Japanese paper called “Washi”. Accordingly, the toner is transferred reliably to the recessed portions of the surface of the paper.
However, it has been recognized that when using regular paper or a coated sheet having a relatively smooth surface as a recording sheet in the image forming apparatus of this kind, improper secondary transfer occurs, which causes easily inadequate image density.
With respect to such a transfer failure, the present inventors have recognized the following. The intermediate transfer belt is interposed between the contact roller and the back surface roller at the secondary transfer nip, and a secondary transfer current flows between the contact roller and the back surface roller. When using a multilayer intermediate transfer belt, the secondary transfer current flows at the boundary between the layers in a thickness direction of the intermediate transfer belt along the circumferential direction of the intermediate transfer belt. As a result, at the secondary transfer nip the secondary transfer current flows not only in the center of the secondary transfer nip at which the nip pressure is the highest, but also at the nip start portion and at the nip end portion. This means that the secondary transfer current flows in the toner image on the intermediate transfer belt in the secondary transfer nip for an extended period of time.
Consequently, a significant amount of charges having a polarity opposite to the charge polarity of toner are injected to the toner, resulting in a decrease in a charge amount of toner Q/M when the toner has a normal polarity. In other words, the secondary transfer ability is degraded, causing inadequate image density.